The present invention relates to a surface-acoustic-wave filter, more particularly to improving the transmission characteristics of a surface-acoustic-wave filter of the resonator type.
Surface-acoustic-wave (hereinafter, SAW) devices in general have an interdigital transducer (IDT) that excites a surface acoustic wave on a piezoelectric substrate. The interdigital transducer can be designed to give the SAW device various characteristics and functions, the filtering function being among the most important. In the past, SAW filters in which surface acoustic waves propagated between two or more interdigital transducers were predominant, but much recent research has focused on SAW filters of the resonator type.
A SAW resonator has a single interdigital transducer, and may have reflectors to keep surface acoustic waves from escaping from the interdigital transducer. The impedance characteristics of a SAW resonator are quite similar to the impedance characteristics of an inductor-capacitor (LC) resonator, so a SAW filter comprising SAW resonators can be designed by classical methods of electrical filter design. In particular, multiple-stage SAW filters can be designed in this way. A simple example is a two-stage SAW filter having a .pi. or T configuration.
In the past, however, inadequate consideration has been given to impedance matching between the different stages of multiple-stage SAW filters. This appears to have been the cause of certain problems that are often seen in the transmission characteristics of SAW filters, and has limited the usefulness of such filters. Further details will be given below.